The present invention relates to an electrically controllable grating and to optical elements having such a grating. Such a grating is suitable for controlling the wavelength of laser light.
It is known to use interdigitated electrodes on electro-optic materials, where application of an electric field causes the formation of a modulating grating within the electro-optic material. The grating can be switched on and off, and may be used to provide beam steering.
GB 1557484 discloses an electro-optical modulator having interdigitated electrodes formed on a surface of a LiNbO3. Application of a voltage to the electrode pattern induces a series of refractive index changes in accordance with the pattern of the electrodes. The pattern of refractive index changes acts as a diffraction grating and the intensities of the orders of the diffracted light vary with the voltage applied to the electrodes.
U.S. Pat. No. 1,516,427 also discloses an electro-optic modulator having interdigitated electrodes formed thereon.
U.S. Pat. No. 3,995,937 discloses a switchable refractive index grating. A series of spaced apart electrodes are arranged in a generally arcuate manner such that the longitudinal axis of each of the electrodes extends towards a common point. The electrodes are arranged in two parallel planes. A waveguide formed in an electro-optic material is positioned intermediate the planes of electrodes. The waveguide follows an arc such that the path of the waveguide is substantially perpendicular to the edges of each electrode. Application of a suitable voltage to the electrodes causes a refractive index grating to be induced within the waveguide. The waveguide is movable radially with respect to the electrodes such that the pitch of the grating can be controlled.
In each of the above, the grating pitch is defined by the electrode pattern and the grating is effectively switched off when no voltage is applied to the electrodes.
According to the present invention, there is provided an electrically controllable grating comprising an electro-optic material and means for applying an electric field to the electro-optic material, characterised in that the electro-optic material has a first portion forming a refractive index grating and comprising an alternating pattern of first regions having a first refractive index in the absence of an electric field and second regions having a second refractive index in the absence of an electric field, the second refractive index being different to the first refractive index.
It is thus possible to vary the refractive index of the material of the refractive index grating and thereby to vary the effective pitch of the grating.
Preferably the means for applying an electric field is arranged to apply an electric field having a component transverse to the refractive index grating.
Preferably first and second electrodes are disposed adjacent the first portion for applying an electric field to the first portion. Advantageously the first and second electrodes are arranged to apply a substantially uniform electric field to the grating.
Advantageously the grating may be included within a laser cavity and be arranged to act as a retro-reflecting grating so as to provide a wavelength controllable laser. The grating may be controlled with a direct current (DC) voltage to perform frequency control suitable for use, for example, in a frequency division multiplexed system. Additionally or alternatively, an alternating voltage, superimposed on a DC voltage (including zero volts) may be applied to the grating to perform frequency multiplexing of the laser.
A plurality of gratings may be provided in parallel so as to enable parallel multiplexing of the laser. Parallel gratings may be formed by provision of segmented electrodes such that a spatially varying electric field may be applied to the first portion. A plurality of lasers may be formed as an array to provide a wavelength division multiplexed light source.
Advantageously the or each laser may further comprise a phase modulator.
Advantageously a grating may be formed in series with at least one optical element on a shared substrate.
Advantageously a grating may be formed in series with a second harmonic generator in order to provide a second harmonic generator which may be electrically controlled to be temperature compensated over a predetermined temperature range.
The grating may advantageously be used as a tunable filter for transmitter and/or receiver elements within an optical communications system.
Furthermore, the grating may be arranged as a surface emitting grating in which the angle of emission is electrically controllable.